Use external __jit_debug_descriptor when IR_EXTERNAL_GDB_ENTRY is defined

[arnaud-lb]

If there are multiple __jit_debug_descriptor symbols, GDB may pick any, and ir_gdb_register_code() has no effect.

[dstogov]

Where do you see the problem? (PHP+linux_x86_64 seems to work fine).
Probably, you are right. I just like to know more about the problem.

[arnaud-lb]

I've had the problem on the same environment (php + linux x86_64). The exact problem I'm having is that GDB is not aware of JIT symbols (so I don't see stubs in backtraces for example).

My understanding is that the main binary's definition should be given precedence by the linker (so ir_gdb_register_code() would use the __jit_debug_descriptor of the php binary), but the behavior I'm observing is that ir_gdb_register_code() uses the symbol of opcache.so. I'm not sure why. Maybe it's due to dlopen().

Building with IR_EXTERNAL_GDB_ENTRY + this patch fixes the issue for me.